In the field of microscopes used for scanning biological samples and the like, it is often necessary to refocus the objective lens rapidly in order to compensate for variations in thickness of a biological sample being inspected. This is done by moving the microscope objective lens relative to a support member so as to adjust the distance between the objective lens and the sample. In most cases these refocusing movements represent small movements of the lens around a reference lens position representative of an average focal plane for the biological sample in question.
There are typically three components that make up the motion of such a lens: a relatively slow or static positioning of the lens at a position of optimum focus; a rapid movement limited in space and time that is used as part of a focusing process; and a medium speed movement of the lens around the position of optimum focus in order to generate images of the sample with depth. For example, the relatively slow or static positioning may involve movements of the order of millimeters on a timescale of tens of seconds; the rapid movement may involve movements of the order of 1 micrometer on a timescale of around 200 microseconds; and the medium motion may involve movements of the order of tens of micrometers over a period of around a few milliseconds to 1 second. These movement scales may vary depending upon the size and setup of the microscope apparatus. Each of these movements may be applied individually or may be applied using a composite control signal. Each of the movements must be highly accurate on a micrometer scale in order to accurately focus an image of the sample.
Controlling these components of motion typically presents a problem in the art. Each component of lens motion required for the generation of a high quality image has different characteristics in time and space and hence a control system adapted to control one component is typically not suitable for the control of another component. For example, to maintain a static position requires a control strategy that is typically not suitable for controlling more rapid limited movement. Hence, there is a requirement for a method and system for controlling the position of a microscope lens which allows the above movements to be performed accurately.